The objective of fiber stock preparation systems is to modify the different ingoing raw materials in such a way that the finished stock finally supplied to the paper machine suits the requirements of the paper machine and of the quality demands put on the produced paper or board. The raw stocks used are the various types of virgin pulps as well as recovered paper grades. They are available in the form of bales, loose material or, in the case of integrated mills, as suspensions. The finished stock is a suspension of defined quality as far as the mixture and characteristics of the fibers, additives, and impurities are concerned. This quality essentially determines paper machine runnability and is the basis for the final paper and board quality.
A stock preparation system consists of several unit processes that are adapted to one another. Their individual efficiency and the reliability of processing machinery depend on suspension properties such as consistency and flake and debris content, on the chemicals entrained and added as well as on the process design itself. So some of the unit processes can be used several times in a preparation process line, others just once. Most of the machines used for performing the individual unit processes have not only one effect but also side effects that may or may not be desirable. The systems differ considerably depending on the raw material used and on the quality of the finished stock required. For instance, fiber design by refining is of central importance in the preparation of chemical pulp. For recovered paper processing the cleanliness of both the individual fibers and the suspension are the main objectives and so the removal of interfering materials is most important.
A stock preparation system comprises essentially three levels (Fig. 4.1):
. • Production level, the direct line from raw material input to the finished stock which may include unit processes with some or all of the following objectives: To break down the raw material into individual fibers, to separate fibers and contaminants, to separate fibers/solids and water, to treat the fibers, and to treat the residual contaminants.
. • Recovery level where fibers and other solids, and water are recovered from the rejects of the separation processes applied in the production level. Again fibers and contaminants as well as solids and water have to be separated.
• Discharge level, for ecological and economic reasons the final rejects are separated into wastewater and residues with high dry content.
The unit processes in fiber preparation and their objectives are:
. • Slushing and deflaking: To break down the fiber raw material into a suspension of individual fibers. Slushing should at least result in a pumpable suspension enabling coarse separation and deflaking if required. In the case of recovered paper, ink particles and other nonpaper particles should be detached from the fibers.
. • Screening: To separate particles from the suspension which differ in size, shape and deformability from the fibers.
. • Fractionation: To separate fiber fractions from each other according to defined criteria such as size or deformability of the fibers.
. • Centrifugal cleaning: To separate particles from the suspension which differ in specific gravity, size and shape from the fibers.
. • Refining: To modify the morphology and surface characteristics of the fibers.
. • Selective flotation: To separate particles from the suspension which differ in surface properties (hydrophobicity) from the fibers.
. • Nonselective flotation: To separate fine and dissolved solids from water.
. • Bleaching: To endow yellowed or brown fibers with the required brightness and luminance.
. • Washing: To separate fine solid particles from suspension (solid/solid separation).
. • Dewatering: To separate water and solids.
. • Dispersing: To reduce the size of dirt specks and stickies (visibility, floatability), to detach ink particles from fibers.
• Mixing and storing: To generate a uniform blend of suspension components, and to prevent suspension components from de-mixing.
Separation processes are of high importance in recovered paper processing as this material contains
. • Additives used in the previous paper production process, such as fillers and dyes, coating components and functional and process chemicals
. • Substances added according to the application such as printing inks, varnishes, coatings, foil laminations, adhesives and waxes
. • Materials mixed with the paper during its life cycle, subsequent collection, and handling in the mill including wires and strings, glass, sand and stones, or paper clips and folders.
Solid particles comprise a wide range of materials with a wide range of properties. Particle characteristics used as separation criteria must clearly differ from fiber characteristics. These include particle size, shape and deformability, density, and surface properties (hydrophobicity). Table 4.1 shows as an example the characteristics “specific gravity and size” for various types of contaminants. Most of these contaminants have to be removed for quality and machine runnability reasons, fillers may be tolerated to a certain degree. Figure 4.2 shows the efficiency scatter of the separation processes vs. particle size. Nonselective flotation (dissolved air flotation) separates small particles very efficiently from water. Washing removes small particles, the efficiency depending on the amount of extracted water. Selective flotation effectively removes particles over a wide size range, but only hydrophobic ones. Screening is most successful in removing larger and cubic particles. For small, flat or deformable particles the separation efficiency is lower. Cleaners have high efficiency in the removal of large particles with densities differing from water and wetted fibers.
The main unit processes, their principles and the machinery applied will be described in the next sections followed by a description of fiber stock preparation systems for different kinds of fiber raw material and final application of the finished stock.